This invention relates to liquid crystal display cells, and more particularly to liquid crystal displays having a light field background with dark displayed characters.
Liquid crystal displays do not emit light. Instead such displays rely on outside illumination transmitted through or reflected off of the device. Problems have been encountered with maintaining consistent spacing between cathode and anode electrodes in liquid crystal display cells. Without consistent spacing, clarity of displayed characters is limited. In order to assure easier readability of liquid crystal displays, it is desirable that the background of the display be relatively light and the characters be dark.
U.S. Pat. No. 4,050,786, Feldman, provides particulate inorganic spacers to achieve uniform spacing between adjacent substrates for liquid crystal displays. Feldman in column 1, line 66 describes his structure as follows: "In exemplary preferred embodiments of the present invention a liquid crystal display is indicated for hermetically containing an electro-optic fluid, such as a nematic liquid crystal, between selectively energized electrical contacts that are regulated in spacing over large areas to provide a controlled potential gradient through the electro-optic fluid, and to eliminate short circuits." Continuing in column 2, line 10: "The conducting surfaces of the substrate are secured in a face-to-face relationship with a spacing provided by a first frit of normally non-transparent glass particles fused to one of the substrates. A denser second frit is deposited on one substrate in a pattern surrounding, but not entering, the pattern formed by the face-to-face electrical conductors and is subsequently fused together and to both substrates bonding them and providing a hermetic seal. An electro-optic fluid, such as a nematic liquid crystal, is selected with a refractive index matching that of the first frit, and is caused to flow into the region surrounding the first frit in positions bounded by the second frit. The fluid frit boundary becomes invisible due to the matched indexes of refraction." While Feldman provides structure for spacing the substrates and sealing around the conductive segments between the substrates, the Feldman structure is not applicable to the preferred display structure designed to achieve a dark display on a light background. Additionally, the utilization of polymeric substances in place of Feldman's glass frits for the spacing and sealing is, as a practical matter, difficult to employ when the display cell is relatively large.
Displays in which the liquid crystal material is host to a guest dichroic dye are known in the art. Heilmeier and Zanoni describe the use of guest dyes as follows: "the orientation of a `guest` dichroic dye (a dye whose optical absorption spectrum is a strong function of the polarization of the incident light with respect to the molecular orientation) is controlled by the orientation of its nematic host in an applied electric field." G. H. Heilmeier and L. A. Zanoni, App. Phys. Lett. 13, 91 (1968). Application of an electric field causes molecules having a general parallel orientation and a positive dielectric anisotropy to align perpendicular to the display cell walls, resulting in the liquid crystal and dye molecules absorbing less light. This appears as a relatively bright character against a darker background.
U.S. Pat. No. 4,128,313, Cole, Jr., et al., describes a light field background display in column 1, line 42 as follows: "In accordance with the invention, a dichroic liquid crystal display having relatively dark characters and a relatively light background is provided by treating at least one of a pair of cell electrodes, each supported upon a transparent substrate, in a manner to achieve a homeotropic alignment over the entire electrode area. Character segment areas are masked on the treated electrode by means of a patterned mask or a layer of photoresist, respectively suitably etched or developed to provide apertures over the character segment area; a parallel-alignment film, of a suitable surfactant or obliquely deposited SiO, is fabricated upon the uncovered areas of the electrode . . . Upon application of a field between character segment electrodes, the parallel alignment is forced to the perpendicular alignment to `extinguish` the character segment by changing the normally dark appearance thereof to the same bright appearance as the background areas." Thus Cole, Jr. provides a light field liquid crystal display but requires that the cell substrates be masked during processing. Masking of the cell substrates is necessary to provide a liquid crystal alignment layer causing parallel alignment of liquid crystal, dye molecules on the electrode segment areas, and a different alignment layer for causing perpendicular alignment on the non-electrode segment areas.